The present invention relates to a process for the preparation of xcex1-amino acids by hydrolysis of hydantoins in the presence of water and of at least one metallic oxide under conditions such that all the starting materials are completely dissolved in the water as a result of high pressure and high temperature and only one further phase is present in the reactor in addition to the solid phase of the metallic oxide.
It is known from U.S. Pat. No. 2,557,920 that xcex1-amino acids are formed by saponification of hydantoins using sodium hydroxide. However, such processes require at least 3 moles of sodium hydroxide per mole of hydantoin. The same is true when potassium hydroxide is used.
DE 19 06 405 describes the hydrolysis of 5-(2-methylmercaptoethyl)-hydantoin using an aqueous solution of alkali carbonate and/or alkali hydrogen carbonate. During the hydrolysis, ammonia and carbon dioxide are constantly removed. Of the alkali carbonates, potassium carbonate is preferred; a molar ratio of hydantoin to alkali of from 1:1 to 1:5 is used. The hydrolysis is carried out under pressure at from 120 to 220xc2x0 C. The alkali methioninate solution is used to liberate D,L-methionine with carbon dioxide; the mother liquor from the separation of the methionine that has crystallized out is used again in the circuit for the hydantoin hydrolysis, optionally with the discharge of from 1 to 2%.
Processes for the preparation of xcex1-amino acids from hydantoins without the simultaneous production of salts are described in JP 03-95 145A and in JP 03-95146A. In those processes, the hydantoins are saponified, with the addition of water and catalysts consisting of metallic oxides (e.g. TiO2, ZrO2), at temperatures from 80 to 220xc2x0 C. with elimination of ammonia. This is carried out discontinuously over a period of from 10 minutes to 10 hours in a stirred autoclave in which there is established a pressure corresponding approximately to the vapour pressure of water at the temperature which has been set. Accordingly, there are at least two phases in the autoclave: a liquid phase and a gas phase.
The processes described in JP 3-95145A and JP 3-95146A, which are carried out batchwise or continuously, lead, with the described dwell times, to the formation of numerous by-products in relatively high concentrations. Yields of max. 69% are mentioned for the preparation of methionine.
Another method for the continuous preparation of methionine without the preparation of a salt as by-product is described in FR-A 27 85 609.
Starting from the amino nitrile of methionine, which is hydrated, with the addition of a ketone (acetone) as homogeneous catalyst, to methionine amide at from 10 to 40xc2x0 C., the methionine amide so obtained is hydrolyzed to methionine at from 100 to 180xc2x0 C. A further possible method of carrying out that hydrolysis consists in a heterogeneously catalyzed reaction at about 100xc2x0 C. and 1 bar, in which catalysts selected from TiO2, TiO2/Al2O3, Nb2O5, Nb2O5xe2x80x94Al2O3, ZnO and ZrO2 can be used. The ammonia formed during the reaction is removed in that process.
The necessary addition of a ketone for the saponification of the amino nitrile of methionine to methionine amide requires further expensive working-up steps.
An object of the invention is to provide a process for the preparation of xcex1-amino acids in which hydantoins are saponified without the formation of waste salts, and the xcex1-amino acids are obtained in a high yield.
The present invention provides a process for the preparation of xcex1-amino acids by hydrolysis of hydantoins of the general formula 
in which
R1, R2: which may be identical or different, and represent hydrogen, alkyl having from 1 to 6 carbon atoms, especially methyl, ethyl, propyl, straight-chain or branched chain; or alkylene radicals having from 1 to 6 carbon atoms which are closed to form a ring when R1 and R2 represent alkylene; or, when R1 or R2 represents alkylene, are bonded to methylthio, mercapto, hydroxyl, methoxy, amino groups or halogen atoms, especially fluorine or chlorine,
R1 or R2: represents a phenyl group which is optionally substituted by methyl, hydroxyl groups or halogens, especially fluorine or chlorine, in the presence of water, ammonia and at least one metallic oxide as catalyst, selected from the group consisting of TiO2, TiO2/Al2O3, Nb2O5/Al2O3, ZnO and ZrO2, in a saponification reactor under conditions in which all the starting materials are completely dissolved in the water and only one further phase is present in the reactor in addition to the solid phase of the metallic oxide.
The terms alkylene or alkylene radical mean to a divalent saturated hydrocarbon radical.
R1 and R2 preferably represent:
R1 hydrogen,
R2 isopropyl, 2-methylpropyl, phenyl or hydrogen,
so that valine, leucine, phenylalanine or glycine are formed from the hydantoins by saponification.
Particular preference is given to the hydantoin in which R1 or R2 represents hydrogen and R2 or R1 represents the 2-methylthioethyl radical, so that methionine is prepared as the product.